It is known to produce alcohols by hydrogenation, or more properly hydrogenolysis, of monoesters of carboxylic acids. The reaction can be summarised as follows: ##STR1## In the book "Catalytic Hydrogenation" by M. Freifelder, published by John Wiley and Sons Inc. (1978), it is stated at page 129 et seq. that the catalyst of choice is barium promoted copper chromite. In "Organic Reactions", Vol. 8, published by John Wiley and Sons Inc. (1954), Chapter I is by Homer Adkins and is entitled "Catalytic Hydrogenation of Esters to Alcohols". It is suggested that a "copper chromite" catalyst is more correctly described as an approximately equimolar combination of cupric oxide and cupric chromite, i.e. CuO,CuCr.sub.2 O.sub.4. Reference may also be had to Kirk-Othmer's "Encyclopedia of Chemical Technology" (Third Edition), Volume 1, page 733 to 739.
Patent specifications describing hydrogenation of esters to yield alcohols include US-A-2040944, US-A-2019414, US-A-2091800, and FR-A-1276722.
It is known to effect hydrogenation of certain esters and diesters in the vapour phase. For example it has been proposed to use a reduced copper oxide/zinc oxide catalyst for effecting hydrogenation of esters in the vapour phase. In this connection attention is directed to GB-B- 2116552. Also of relevance is WO-A-90/8121.
It is further known to produce diols, such as butane-1,4-diol, by catalytic hydrogenation of esters of dicarboxylic acids, such as a dimethyl or diethyl ester of maleic acid, fumaric acid, succinic acid, or a mixture of two or more thereof. Such processes are described, for example, in GB-A-1454440, GB-A-1464263, DE-A-2719867, US-A-4032458, and US-A-4172961.
Production of butane-1,4-diol by vapour phase hydrogenation of a diester, typically a dialkyl ester, of a C.sub.4 dicarboxylic acid selected from maleic acid, fumaric acid, succinic acid, and a mixture of two or more thereof has been proposed. In such a process the diester is conveniently a di-(C.sub.1 to C.sub.4 alkyl) ester, such as dimethyl or diethyl maleate, fumarate, or succinate. A further description of such a process can be found in US-A-4584419, EP-A-0143634, WO-A-86/03189, WO-A-86/07358, and WO-A88/00937.
In all of the above-mentioned vapour phase processes the esters or diesters all have a vapour pressure which is high compared to the vapour pressure of dimethyl 1,4-cyclohexanedicarboxylate and 1,4-cyclohexanedimethanol.
Another commercially important diol is 1,4-cyclohexanedimethanol. This compound is used to prepare highly polymeric linear condensation polymers by reaction with terephthalic acid and is useful as an intermediate in the preparation of certain polyester and polyester amides. The use of 1,4-cyclohexanedimethanol for such purposes is disclosed in, for example, US-A-2901466. This document teaches that the trans-isomer of polycyclohexylenedimethylene terephthalate has a higher melting point range (315.degree.-320.degree. C.) than the corresponding cis-isomer (260.degree.-267.degree. C.).
One method for preparing 1,4-cyclohexanedimethanol (hexahydroterephthalyl alcohol) involves the hydrogenation of diethyl 1,4-cyclohexanedicarboxylate (diethyl hexahydroterephthalate) in a slurry phase reactor in the presence of a copper chromite catalyst at a pressure of 3000 psia (about 206.84 bar) and a temperature of 255.degree. C., as is described in Example 3 of US-A-2105664. The yield is said to be 77.5%.
The hydrogenation of dimethyl 1,4-cyclohexanedicarboxylate (DMCD) to 1,4-cyclohexanedimethanol (CHDM) is shown below in equation (1): ##STR2##
Several processes for the production of 1,4-cyclohexanedimethanol and related alcohols have been published since the issuance of US-A-2105664 in 1938. These processes have, almost exclusively, focused on methods for performing the above hydrogenation reaction in the liquid phase. Advances have been reported in the general areas of cis-/trans- isomer selectivity (US-A-2917549, GB-A-988316 and US-A-4999090), catalyst type (GB-A-988316 and US-A3334149) and conditions of plant operation (US-A-5030771).
In the hydrogenation of dimethyl 1,4-cyclohexanedicarboxylate the two geometrical isomers of CHDM produced are: ##STR3##
The resulting 1,4-cyclohexanedimethanol product is a mixture of these two isomers which have different melting points. As reported on page 9 of the book "Fiber Chemistry" edited by Menachem Lewis and Eli M. Pearce, published by Marcel Dekker, Inc.: "Both the alicyclic ester [i.e. dimethyl 1,4-cyclohexanedicarboxylate] and the alicyclic diol [i.e. 1,4-cyclohexanedimethanol] exist in two isomeric forms, cis . . . and trans . . . , that are not interconvertible without bond rupture". The passage continues later: "Control of the [cis-:trans-] ratio is important [in 1,4-cyclohexanedimethanol] since many polymer and fiber properties depend on it".
The cis-isomer of 1,4-cyclohexanedimethanol has a melting point of 43.degree. C. and the trans has a melting point of 67.degree. C. The higher melting point trans-isomer is often preferred over the cis-isomer for use as a reagent in the preparation of polyester and polyester-amides if a high melting point for such materials is considered desirable. As noted above, the trans-isomer of a typical polyester, such as trans-polycyclohexylmethyl terephthalate, has a higher melting point than the cis-isomer. Hence, for example, US-A-5124435 discloses a polyester copolymer, the 1,4-cyclohexanedimethanol content of which has a trans-isomer content of at least 80 mole %, and which has a high heat resistance. The preferment of trans-1,4-cyclohexanedimethanol over cis-1,4-cyclohexanedimethanol is also discussed in US-A-2917549, in US-A-4999090 and in GB-A988316.
A liquid phase process for the production of 1,4-cyclohexanedimethanol by plural stage hydrogenation of dimethyl terephthalate is described in US-A-3334149. This utilises a palladium catalyst to effect hydrogenation of dimethyl terephthalate to dimethyl 1,4-cyclohexanedicarboxylate, followed by use of a copper chromite catalyst in the liquid phase to catalyse the hydrogenation of that diester to 1,4-cyclohexanedimethanol. In the procedure described in Example 1 of that patent specification a residence time of about 40 to 50 minutes is used in the second stage of this process. The activity of the copper chromite catalysts recommended in US-A-3334149 is such that long residence times are required.
In a liquid phase process for the production of 1,4-cyclohexanedimethanol, such as is disclosed in US-A-3334149, the trans-:cis- isomer ratio of the product 1,4-cyclohexanedimethanol will tend towards an equilibrium value. This equilibrium value has been reported variously and may lie between about 2.57:1 (trans-:cis- 1,4-cyclohexanedimethanol) (as reported in GB-A-988316) and about 3:1 (as reported in US-A-2917549). However, the starting material, dimethyl 1,4-cyclohexanedicarboxylate, is generally commercially obtainable as a mixture of cis- and trans-isomers wherein there is a preponderance of the cis-isomer. Thus in a typical commercial grade of dimethyl 1,4-cyclohexanedicarboxylate the trans-:cis- isomer ratio is from about 0.5:1 to about 0.6:1.
Attempts to deal with the problem of the presence of an excess of the less desirable cis-1,4-cyclohexanedimethanol isomer in any process for 1,4-cyclohexanedimethanol manufacture have focused on the isomerisation of the cis-isomer of cyclohexanedimethanol to the trans-isomer thereof.
US-A-2917549 discloses a process for isomerising cis-1,4-cyclohexanedimethanol to trans-1,4-cyclohexanedimethanol which comprises heating cis-1,4-cyclohexanedimethanol at a temperature of at least 200.degree. C. in the presence of an alkoxide of a lower atomic weight metal such as lithium, sodium, potassium, calcium or aluminium. However, the process of US-A-2917549 necessarily involves a two-stage process wherein the initial cis-/trans- 1,4-cyclohexanedimethanol hydrogenation product is recovered from the hydrogenation zone and subjected to temperatures in excess of 200.degree. C. in the presence of a metal alkoxide catalyst under an atmosphere of nitrogen. The capital and operational costs associated with a plant designed to carry out the process taught in US-A-2917549 would be undesirably high. Another disadvantage of such a plant is the associated hazard relating to the use of metal alkoxides as catalysts in the isomerisation zone. Such catalysts are required to effect the isomerisation, which is reported not to occur under typical hydrogenation conditions using hydrogenation catalysts such as copper/chrome or Raney nickel catalysts, according to the teaching of Example 11 of US-A-2917549. Furthermore, steps would be required to prevent product contamination by the metal alkoxide catalyst.
US-A-4999090 discloses a process for the isomerisation of cis-1,4-cyclohexanedimethanol by distillation in the presence of an alkali metal hydroxide or alkoxide at a temperature of between 150.degree. C. and 200.degree. C. and at a pressure of between 1 mm Hg and 50 mm Hg (between 1.33 millibar and 66.5 millibar). This process has very similar disadvantages to those of US-A-2917549.
GB-A-988316 teaches a process for the preparation of trans-1,4-cyclohexanedimethanol in which a mixture of cis- and trans-isomers of dimethyl hexahydroterephthalate (i.e. dimethyl 1,4-cyclohexanedicarboxylate) is hydrogenated at elevated temperature and pressure in the presence of a Cu/Zn catalyst. Trans-1,4-dimethylolcyclohexane (i.e. trans-1,4-cyclohexanedimethanol) is separated by crystallisation from the reaction product and then the residual product, now enriched in cis-1,4-cyclohexanedimethanol, is recycled to the hydrogenation zone whereupon it undergoes isomerisation to a cis-/trans- 1,4-cyclohexanedimethanol mixture. The recycle procedure may be repeated to obtain a 1,4-cyclohexanedimethanol product containing the trans-isomer in substantial excess. However, the process according to GB-A-988316 is more preferably operated under conditions such that recycled cis-isomer enriched product is combined with fresh dimethyl 1,4-cyclohexanedicarboxylate feed on re-entry to the hydrogenation zone. The effectiveness of recycling the cis-isomer to the hydrogenation zone is largely a result of the dual function of the copper/zinc catalyst which possesses both a hydrogenating and an isomerising catalytic action. As would be expected from thermodynamic principles, the isomerising action is most effective when a mixture containing a preponderance of the cis-isomer is recycled to the hydrogenation zone. However, recycling the cis-isomer in this way is acknowledged to cause a new problem, that of the formation of unwanted by-products, such as 1-methyl-4-hydroxymethylcyclohexane, which may be formed by operating the hydrogenation reaction under too severe conditions. To minimise the formation of such by-products, the hydrogenation zone may be operated under "relatively mild conditions", according to the teaching of GB-A-988316 (see, for example page 2, lines 55 to 79 of GB-A-988316). However, such mild conditions reduce the achieved conversion of dimethyl 1,4-cyclohexanedicarboxylate with the result that, for any one pass through the hydrogenation zone, a significant quantity of dimethyl hexahydroterephthalate (dimethyl 1,4-cyclohexanedicarboxylate) remains unconverted. By the term "relatively mild conditions" is meant a temperature of at least 200.degree. C., preferably between 240.degree. C. and 300.degree. C., and a pressure of 200 to 300 atmospheres (202.65 bar to 303.98 bar), according to page 2, lines 26 to 32 of GB-A-988316. The use of such high pressures at these elevated temperatures can be hazardous, besides requiring reactors with thick walls and flanges of special alloy constructed to withstand such extreme pressures. Hence it is expensive to construct a plant to operate at pressures as high as envisaged in GB-A-988316. Furthermore it is potentially hazardous to operate a plant operating at 200 atmospheres (202.65 bar) or above, as well as being very expensive, not only in terms of the capital cost of the plant but also with regard to operating costs. A substantial proportion of this capital cost is associated with the rigorous safety precautions that must be taken when operating a high pressure conventional commercial scale hydrogenation plant. It is also expensive to compress gaseous streams to such high pressures and to circulate them through the plant.
Although there is a passing reference (see page 1, line 84 of GB-A-988316) to use of "the gaseous phase", even at temperatures of 300.degree. C. both cis- and trans- dimethyl hexahydroterephthalate would be in the liquid phase at pressures of 200 to 300 atmospheres (202.65 bar to 303.98 bar) at the hydrogen:ester ratio envisaged in the Examples. Thus in each of the Examples of GB-A-988316 liquid phase conditions are used. According to Example 4, which uses a feed mixture containing dimethyl hexahydroterephthalate (i.e. 1,4-dimethyl cyclohexanedicarboxylate), and methanol such as might be used in a recycling process, the isomers present in the diol in the hydrogenation product are stated to represent an equilibrium mixture of about 72% of the trans- and about 28% of the cis-isomer, i.e. a trans-:cis-ratio of about 2.57:1.
The hydrogenation of an ester, diester or lactone feedstock is generally an exothermic reaction. In a liquid phase reaction the feedstock is normally diluted with an inert diluent, conveniently with recycled product hydroxylic compound, and the catalyst is wholly wetted with liquid. The diluent acts as a heat sink and helps to prevent the danger of damage to the catalyst due to the exothermic nature of the hydrogenation reaction.
In a typical vapour phase hydrogenation process the unsaturated organic compound, i.e. the ester, diester or lactone, is normally vaporised directly into a hot hydrogen-containing gas to give a vaporous mixture, which may be heated further or diluted with more hot hydrogen-containing gas in order to raise its temperature above the dew point. It is normally essential to ensure that the vaporous mixture in contact with the catalyst is above its dew point, i.e. above that temperature at which a mixture of gases and vapour just deposits liquid as a fog or a film. This dew point liquid will normally contain all the condensable components of the vapour phase, as well as dissolved gases, in concentrations that satisfy the usual vapour/liquid criteria. It may include the starting material, an intermediate product, a by-product and/or the final hydrogenation product. Generally the process is operated so that the temperature of the vaporous feed mixture is above its dew point, for example about 5.degree. C. to about 10.degree. C. above its dew point. Moreover it is desirable to prevent contact of droplets of liquid with the catalyst, particularly droplets which are rich in the unsaturated organic compound, because damage to the catalyst may result from loss of mechanical strength, from formation of hot spots on the surface of the catalyst or in the pores of the catalyst, due to the exothermic nature of the reaction, leading possibly to sintering and thereby to loss of chemically effective catalyst surface area (particularly in the case of copper-containing catalysts), or from disintegration of the catalyst pellets possibly as a result of explosive vaporisation within the pores of the pellets.